Phase shift mask blank, phase shift mask, and blank preparing method

ABSTRACT

In a phase shift mask blank comprising a transparent substrate and a phase shift film deposited thereon and having a phase shift of 150-200° with respect to sub-200 nm light, the phase shift film is composed of a silicon base material consisting of silicon, nitrogen and optionally oxygen, has a thickness of up to 70 nm, and provides a warpage change of up to 0.2 μm in a central region of a surface of the substrate before and after the deposition of the phase shift film on the substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2015-072766 filed in Japan on Mar. 31, 2015, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a phase shift mask for use in the microfabrication of semiconductor integrated circuits or the like, a phase shift mask blank, and a method for preparing the mask blank.

BACKGROUND ART

In the field of semiconductor technology, research and development efforts are continued for further miniaturization of pattern features. Recently, as advances including miniaturization of circuit patterns, thinning of interconnect patterns and miniaturization of contact hole patterns for connection between cell-constituting layers are in progress to comply with higher integration density of LSIs, there is an increasing demand for the micropatterning technology. Accordingly, in conjunction with the technology for manufacturing photomasks used in the exposure step of the photolithographic microfabrication process, it is desired to have a technique of forming a more fine and accurate circuit pattern or mask pattern.

In general, reduction projection is employed when patterns are formed on semiconductor substrates by photolithography. Thus the size of pattern features formed on a photomask is about 4 times the size of pattern features formed on a semiconductor substrate. In the current photolithography technology, the size of circuit patterns printed is significantly smaller than the wavelength of light used for exposure. Therefore, if a photomask pattern is formed simply by multiplying the size of circuit pattern 4 times, the desired pattern is not transferred to a resist film on a semiconductor substrate due to optical interference and other effects during exposure.

Sometimes, optical interference and other effects during exposure are mitigated by forming the pattern on a photomask to a more complex shape than the actual circuit pattern. Such a complex pattern shape may be designed, for example, by incorporating optical proximity correction (OPC) into the actual circuit pattern. Also, attempts are made to apply the resolution enhancement technology (RET) such as modified illumination, immersion lithography or double exposure (or double patterning) lithography, to meet the demand for miniaturization and higher accuracy of patterns.

The phase shift method is used as one of the RET. The phase shift method is by forming a pattern of film capable of phase reversal of approximately 180 degrees on a photomask, such that contrast may be improved by utilizing optical interference. One of the photomasks adapted for the phase shift method is a halftone phase shift mask. Typically, the halftone phase shift mask includes a substrate of quartz or similar material which is transparent to exposure light, and a mask pattern of halftone phase shift film formed on the substrate, capable of providing a phase shift of approximately 180 degrees and having an insufficient transmittance to contribute to pattern formation. As the halftone phase shift mask, Patent Document 1 (JP-A H07-140635) proposes a mask having a halftone phase shift film of molybdenum silicide oxide (MoSiO) or molybdenum silicide oxynitride (MoSiON).

For the purpose of forming finer images by photolithography, light of shorter wavelength is used as the light source. In the currently most advanced stage of lithography process, the exposure light source has made a transition from KrF excimer laser (248 nm) to ArF excimer laser (193 nm). The lithography using ArF excimer laser light of greater energy was found to cause damages to the mask, which were not observed with KrF excimer laser light. One problem is that on continuous use of the photomask, foreign matter-like growth defects form on the photomask. These growth defects are also known as “haze”. The source of haze formation was formerly believed to reside in the growth of ammonium sulfate crystals on the mask pattern surface. It is currently believed that organic matter participates in haze formation as well.

Some approaches are known to overcome the haze problem. With respect to the growth defects formed on the photomask upon long-term irradiation of ArF excimer laser light, for example, Patent Document 2 (JP-A 2008-276002) describes that if the photomask is cleaned at a predetermined stage, then it can be continuously used.

As the exposure dose of ArF excimer laser light irradiated for pattern transfer increases, the photomask is given damage different from haze; and the size of the mask pattern changes in accordance with the cumulative irradiation energy dose, as reported in Non-Patent Document 1 (Thomas Faure et al., “Characterization of binary and attenuated phase shift mask blanks for 32 nm mask fabrication,” Proc. of SPIE, vol. 7122, pp 712209-1 to 712209-12). This problem is that as the cumulative irradiation energy dose increases during long-term irradiation of ArF excimer laser light, a layer of a substance which is considered to be an oxide of the pattern material grows outside the film pattern, whereby the pattern width changes. It is also reported that the mask once damaged cannot be restored by cleaning with AMP (aqueous ammonia/hydrogen peroxide) as used in the above-mentioned haze removal or with SPM (sulfuric acid/hydrogen peroxide). It is believed that the damage source is utterly different.

Non-Patent Document 1 points out that upon exposure of a circuit pattern through a halftone phase shift mask which is the mask technology useful in expanding the depth of focus, substantial degradation is induced by pattern size variation resulting from alteration of a transition metal/silicon base material film such as MoSi base material film by irradiation of ArF excimer laser light (this degradation is referred to as “pattern size variation degradation”). Then, in order to use an expensive photomask over a long period of time, it is necessary to address the pattern size variation degradation by irradiation of ArF excimer laser light.

As pointed out in Non-Patent Document 1, the pattern size variation degradation by irradiation of short wavelength light, typically ArF excimer laser light does scarcely occur when light is irradiated in a dry air atmosphere. Exposure in a dry air atmosphere is regarded as a new approach for inhibiting the pattern size variation degradation. However, the control of a dry air atmosphere adds an extra unit to the exposure system and gives rise to electrostatic and other problems to be managed, leading to an increased expense. Under the circumstances, it is necessary to enable long-term exposure in a common atmosphere that does not need complete removal of humidity (typically having a humidity of around 50%).

The photomasks used in the lithography using ArF excimer laser light as light source include halftone phase shift masks having a halftone phase shift film of a silicon base material containing a transition metal, typically molybdenum. This silicon base material is mainly composed of a transition metal and silicon, and further contains oxygen and/or nitrogen as light element (e.g., Patent Document 1). Suitable transition metals used include Mo, Zr, Ta, W, and Ti. Among others, Mo is most often used (e.g., Patent Document 1). Sometimes a second transition metal is added (e.g., Patent Document 3). For the light-shielding film, silicon base materials containing a transition metal, typically molybdenum are also used. However, when a photomask using such transition metal-containing silicon base material is exposed to a large dose of high-energy radiation, the mask undergoes significant pattern size variation degradation by irradiation of high-energy radiation. Then the service lifetime of the photomask is shorter than the requirement.

It is a serious problem that when a photomask pattern on a halftone phase shift mask is irradiated with short-wavelength light, typically ArF excimer laser light, the photomask pattern for exposure experiences a variation of line width, that is, “pattern size variation degradation.” The permissible threshold of pattern width differs with the type of photomask pattern, especially the pattern rule applied thereto. If variations are small, the mask may be further used by correcting the exposure conditions and resetting the irradiation conditions of an exposure system. For example, in the lithography for forming semiconductor circuits complying with the pattern rule of 22 nm, the variation of mask pattern line width must fall within approximately ±5 nm. However, if a pattern width variation is large, there is a possibility that the variation has an in-plane distribution on the photomask. Also in the further miniaturization technology, an auxiliary pattern having an ultrafine size of less than 100 nm is formed on the mask. For the purpose of pattern miniaturization on these masks and from the aspect of an increase of mask processing cost by complication of mask pattern, there is a need for a halftone phase shift mask film which experiences minimal pattern size variation degradation and allows for repeated exposure.

On use of a halftone phase shift mask blank in the halftone phase shift mask producing process, if foreign deposits are on the mask blank, they cause defects to the pattern. To remove foreign deposits, the halftone phase shift mask blank is cleaned many times during the mask producing process. Further, when the halftone phase shift mask thus produced is used in the photolithography process, the mask is also repeatedly cleaned even if the mask itself is free of pattern defects, for the reason that if foreign deposits settle on the mask during the photolithography process, a semiconductor substrate which is patterned using that mask eventually bears pattern-transfer failures.

For removing foreign deposits from the halftone phase shift mask blank or mask, chemical cleaning is applied in most cases, using SPM, ozone water or AMP. SPM is a sulfuric acid/hydrogen peroxide mixture which is a cleaning agent having strong oxidizing action. Ozone water is water having ozone dissolved therein and used as a replacement of SPM. AMP is an aqueous ammonia/hydrogen peroxide mixture. When the mask blank or mask having organic foreign deposits on its surface is immersed in the AMP cleaning liquid, the organic foreign deposits are liberated and removed from the surface under the dissolving action of ammonia and the oxidizing action of hydrogen peroxide.

Although the chemical cleaning with such chemical liquid is necessary for removing foreign deposits such as particles and contaminants on the halftone phase shift mask blank or mask, the chemical cleaning can damage the halftone phase shift film on the mask blank or mask. For example, if the surface of a halftone phase shift film is altered by chemical cleaning, the optical properties that the film originally possesses can be changed. In addition, chemical cleaning of the halftone phase shift mask blank or mask is repeatedly carried out. It is thus necessary to minimize any property change (e.g., phase shift change) of the halftone phase shift film during every cleaning step.

CITATION LIST

-   Patent Document 1: JP-A H07-140635 -   Patent Document 2: JP-A 2008-276002 (U.S. Pat. No. 7,941,767) -   Patent Document 3: JP-A 2004-133029 -   Patent Document 4: JP-A 2007-033469 -   Patent Document 5: JP-A 2007-233179 -   Patent Document 6: JP-A 2007-241065 -   Non-Patent Document 1: Thomas Faure et al., “Characterization of     binary and attenuated phase shift mask blanks for 32 nm mask     fabrication,” Proc. of SPIE, vol. 7122, pp 712209-1 to 712209-12

SUMMARY OF INVENTION

With respect to the phase shift film, a thinner film is advantageous for pattern formation and effective for reducing 3D effect. Thus a thinner film is required in order for photolithography to form a finer size pattern.

As alluded to previously (Patent Document 1), a film containing molybdenum and silicon is mainly used as the phase shift film. Regrettably, the transition metal-containing film experiences a noticeable pattern size variation degradation by irradiation of ArF excimer laser light of wavelength 193 nm. Since cleaning is repeated during the mask manufacture process or mask use, the transition metal-containing film also undergoes noticeable changes of optical properties on every cleaning. These problems may be solved by forming the phase shift film from a transition metal-free silicon base material. Nevertheless, the phase shift film formed of a transition metal-free silicon base material, especially nitrogen-containing silicon base material contains a substantial stress therein. When a phase shift mask is manufactured from this phase shift mask blank by patterning the phase shift film, a flatness change occurs before and after the pattern formation of the phase shift film, resulting in misalignment of the pattern on the phase shift mask, that is, a low dimensional accuracy.

An object of the invention is to provide a phase shift mask blank having a phase shift film which is thin enough to be advantageous for pattern formation and 3D effect reduction while maintaining a necessary phase shift for the phase shift function to comply with pattern size miniaturization, and is improved in quality and dimensional control so as to minimize pattern misalignment and dimensional accuracy lowering during the step of patterning the phase shift film to form a phase shift mask. Another object is to provide a phase shift mask and a method for preparing the phase shift mask blank.

Aiming to develop a phase shift film which has a reduced thickness and is minimized in pattern misalignment and dimensional accuracy lowering during the step of patterning the phase shift film to form a phase shift mask while maintaining a necessary phase shift for the phase shift function, the inventors made a study on a transition metal-free phase shift film. It has been found that a desired phase shift film is formed on a transparent substrate by sputter depositing at least one layer of a silicon base material consisting of silicon, nitrogen and optionally oxygen and heat treating the film at 400° C. or above for at least 5 minutes. Then the phase shift film has a thickness of up to 70 nm and a phase shift of 150 to 200° with respect to light of wavelength up to 200 nm light and provides a warpage change (ΔTIR) of up to 0.2 μm as absolute value in a central region of 142 mm squares on a surface of the substrate between before the deposition of the phase shift film on the substrate and the presence of the phase shift film on the substrate, or a warpage change (ΔTIR) of up to 0.2 μm as absolute value in a central region of 142 mm squares on a surface of the substrate between the presence of the phase shift film on the substrate and after complete removal of the phase shift film from the substrate by etching.

Once the phase shift film is constructed as above, there is obtained a phase shift mask blank which can be advantageously processed into a phase shift film pattern including a main photomask pattern with a width of about 100 to 200 nm, which is necessary in photolithography for forming a pattern with a half pitch of up to 50 nm on a processable substrate such as silicon wafer, using exposure light of wavelength up to 200 nm. The phase shift film is minimized in pattern misalignment and improved, in dimensional accuracy during the step of patterning the phase shift film.

Accordingly, in one aspect, the invention provides a phase shift mask blank comprising a transparent substrate of 152 mm squares and 6.35 mm thick and a phase shift film deposited on the substrate and having a phase shift of 150 to 200° with respect to light of wavelength up to 200 nm. The phase shift film includes at least one layer composed of a silicon base material consisting of silicon and nitrogen or a silicon base material consisting of silicon, nitrogen and oxygen, has a thickness of up to 70 nm, and provides a warpage change (ΔTIR) of up to 0.2 μm as absolute value in a central region of 142 mm squares on a surface of the substrate between before the deposition of the phase shift film on the substrate and the presence of the phase shift film on the substrate.

In another aspect, the invention provides a phase shift mask blank comprising a transparent substrate of 152 mm squares and 6.35 mm thick and a phase shift film deposited on the substrate and having a phase shift of 150 to 200° with respect to light of wavelength up to 200 nm. The phase shift film includes at least one layer composed of a silicon base material consisting of silicon and nitrogen or a silicon base material consisting of silicon, nitrogen and oxygen, has a thickness of up to 70 nm, and provides a warpage change (ΔTIR) of up to 0.2 μm as absolute value in a central region of 142 mm squares on a surface of the substrate between the presence of the phase shift film on the substrate and after complete removal of the phase shift film from the substrate by etching.

In a preferred embodiment, the phase shift film is a halftone phase shift film composed of a silicon base material consisting of silicon and nitrogen and having a transmittance of 3% to less than 20% with respect to light of wavelength up to 200 nm.

In another preferred embodiment, the phase shift film is a halftone phase shift film composed of a silicon base material consisting of silicon, nitrogen and oxygen and having a transmittance of at least 20% with respect to light of wavelength up to 200 nm.

The mask blank may further comprise a second film on the phase shift film, the second film being a single layer or a multilayer film composed of a chromium-containing material. Typically, the second film is a light-shielding film, a combination of light-shielding film and antireflective film, or an auxiliary film which functions as a hard mask during pattern formation of the phase shift film.

The mask blank may further comprise a third film on the second film, the third film being a single layer or a multilayer film composed of a silicon-containing material. In a preferred embodiment, the second film is a light-shielding film or a combination of light-shielding film and antireflective film, and the third film is an auxiliary film which functions as a hard mask during pattern formation of the second film. In another preferred embodiment, the second film is an auxiliary film which functions as a hard mask during pattern formation of the phase shift film and as an etch stopper during pattern formation of the third film, and the third film is a light-shielding film or a combination of light-shielding film and antireflective film.

The mask blank may further comprise a fourth film on the third film, the fourth film being a single layer or a multilayer film composed of a chromium-containing material. In a preferred embodiment, the second film is an auxiliary film which functions as a hard mask during pattern formation of the phase shift film and as an etch stopper during pattern formation of the third film, the third film is a light-shielding film or a combination of light-shielding film and antireflective film, and the fourth film is an auxiliary film which functions as a hard mask during pattern formation of the third film.

In any embodiments, the phase shift film has been sputter deposited and heat treated at 400° C. or above for at least 5 minutes.

Also contemplated herein is a phase shift mask prepared from the phase shift mask blank defined above.

In a further aspect, the invention provides a method for preparing a phase shift mask blank comprising the steps of sputter depositing a phase shift film on a transparent substrate, and heat treating the phase shift film on the substrate at 400° C. or above for at least 5 minutes. The depositing step includes one or both of step (A) of depositing a layer composed of a silicon base material consisting of silicon and nitrogen on a transparent substrate by using a sputtering system with a chamber and at least one silicon target, and feeding argon gas and nitrogen gas into the chamber, and step (B) of depositing a layer composed of a silicon base material consisting of silicon, nitrogen and oxygen on a transparent substrate by using a sputtering system with a chamber and at least one silicon target, and feeding argon gas, and at least one gas selected from nitrogen gas, oxygen gas and nitrogen oxide gas into the chamber.

Advantageous Effects of Invention

The phase shift mask blank of the invention has a phase shift film which is thin enough to be advantageous for photomask pattern formation, improved in quality and dimensional control so as to minimize pattern misalignment and dimensional accuracy lowering during the step of patterning the phase shift film to manufacture a phase shift mask, and maintains a necessary phase shift for the phase shift function. Using the phase shift mask, lithography exposure complying with the requirements of pattern miniaturization and patterning accuracy becomes possible.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are cross-sectional views of one exemplary phase shift mask blank and phase shift mask of the invention, respectively.

FIGS. 2A, 2B and 2C are cross-sectional views of further embodiments of the phase shift mask blank of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention pertains to a phase shift (photo)mask blank comprising a transparent substrate and a phase shift film formed thereon. The transparent substrate is typically a quartz substrate. Preference is given to transparent substrates of 6 inch squares and 25 mil thick, known as 6025 substrate, as prescribed in the SEMI standards, or transparent substrates of 152 mm squares and 6.35 mm thick when expressed in the SI units. The phase shift film may be a single layer structure or a multilayer structure (i.e., two or more layers). The phase shift (photo)mask has a (photo)mask pattern of a phase shift film. The term “phase shift film” is inclusive of a halftone phase shift film. Accordingly, the phase shift mask blank is inclusive of a halftone phase shift mask blank, and the phase shift mask is inclusive of a halftone phase shift mask.

FIG. 1A is a cross-sectional view of a phase shift mask blank in one embodiment of the invention. The phase shift mask blank 100 includes a transparent substrate 10 and a phase shift film 1 formed thereon. FIG. 1B is a cross-sectional view of a phase shift mask in one embodiment of the invention. The phase shift mask 101 includes a transparent substrate 10 and a phase shift film pattern 11 thereon.

The phase shift film may be composed of a single layer or multiple layers as long as a phase shift and a transmittance necessary for the phase shift function are met. In the case of multilayer structure, the film is preferably composed of multiple layers including an antireflective function layer so that the overall film may meet a predetermined surface reflectance as well as the necessary phase shift and transmittance.

In either of the single layer and multilayer structure, each layer may be a uniform layer or a compositionally graded layer (i.e., a layer whose composition varies continuously in thickness direction). In the case of multilayer structure, the phase shift film may be a combination of two or more layers selected from layers composed of different constituents and layers composed of identical constituents in different compositional ratios. Where the film is composed of three or more layers, identical layers may be included as long as they are not contiguous to each other.

Since phase shift masks are used in the photolithography using exposure light of wavelength up to 200 nm, typically ArF excimer laser light (wavelength 193 nm), the phase shift film should provide a predetermined phase shift and a predetermined transmittance with respect to the exposure light at a predetermined thickness.

The (overall) thickness of the phase shift film should preferably be up to 70 nm, and more preferably up to 62 nm, because a thinner film facilitates to form a finer pattern. The lower limit of the film thickness is set in the range where the desired optical properties are obtained relative to light of wavelength up to 200 nm. Specifically, the film thickness is set at least 40 nm, though the lower limit is not critical.

The phase shift of the phase shift film with respect to exposure light is such that a phase shift between the exposure light transmitted by a region of phase shift film (phase shift region) and the exposure light transmitted by a neighboring region where the phase shift film is removed, causes interference of exposure light at the boundary whereby contrast is increased. Specifically the phase shift is 150 to 200 degrees. Although ordinary phase shift films are set to a phase shift of approximately 180°, it is possible from the standpoint of contrast enhancement to adjust the phase shift below or beyond 180°. For example, setting a phase shift of smaller than 180° is effective for forming a thinner film. It is a matter of course that a phase shift closer to 180° is more effective because a higher contrast is available. In this regard, the phase shift is preferably 160 to 190°, more preferably 175 to 185°, and most preferably approximately 180°.

In one embodiment, the phase shift film provides a warpage change (ΔTIR) of up to 0.2 μm as absolute value in a central region of 142 mm squares on a surface of the substrate before and after the deposition of the phase shift film on the substrate, i.e., between the absence and the presence of the phase shift film. In another embodiment, the phase shift film provides a warpage change (ΔTIR) of up to 0.2 μm as absolute value in a central region of 142 mm squares on a surface of the substrate between the presence of the phase shift film on the substrate and after complete removal of the phase shift film from the substrate by etching. Now that the phase shift film is designed so as to minimize a warpage change, there is obtained a phase shift mask blank having a phase shift film which is minimized in pattern misalignment and dimensional accuracy lowering during the step of patterning the phase shift film to form a phase shift mask.

The central region of 142 mm squares on the surface of a transparent substrate is defined as a range that extends inward from a position spaced 5 mm apart from each side of the surface of a transparent substrate of 152 mm squares on which a phase shift film is deposited. This range corresponds to a region of a phase shift mask where a photomask pattern used for exposure is formed. The warpage (or sori) of the bare transparent substrate and the phase shift film-bearing transparent substrate is a flatness as prescribed by TIR (total indicator reading) on measurement of the surface topography of the substrate by surface topography analyzing system or flatness tester. When the surface topography of an identical substrate is measured on the assumption that the height at the center of the substrate is an origin in height direction, a warpage change (ΔTIR) is defined as the maximum or minimum of a change at each coordinate in the substrate surface (plane) between the presence and the absence of the film on the substrate. The warpage and warpage change may be measured and computed by a commercial flatness tester, for example, Tropel® UltraFlat 200 Mask (Corning Inc.).

In the embodiment wherein the phase shift film is a halftone phase shift film, the phase shift film has a transmittance of exposure light which is preferably at least 3%, more preferably at least 5%, and up to 40%, more preferably up to 30%.

Particularly when the phase shift film is composed of a silicon base material consisting of silicon and nitrogen, it may be formed as a halftone phase shift film having a transmittance of 3% to less than 20% with respect to sub-200 nm wavelength light by adjusting the contents of silicon and nitrogen in the film. Also when the phase shift film is composed of a silicon base material consisting of silicon, nitrogen and oxygen, it may be formed as a halftone phase shift film having a transmittance of at least 20% with respect to sub-200 nm wavelength light by adjusting the contents of silicon, nitrogen and oxygen in the film.

When the phase shift film is a single layer, the overall single layer, or when the film is a multilayer film, at least one layer, especially the overall layers (excluding a surface oxidized layer if any) should preferably have a refractive index n of at least 2.4, more preferably at least 2.5, and even more preferably at least 2.6 with respect to the exposure light. By reducing the oxygen content of a phase shift film, preferably by eliminating oxygen, or by eliminating transition metal from the film, the refractive index n of the film can be increased while maintaining the predetermined transmittance, and the thickness of the film can be reduced while maintaining the necessary phase shift for the phase shift function. The refractive index n becomes higher as the oxygen content is lower, and the necessary phase shift is available from a thinner film as the refractive index n is higher.

When the phase shift film is a single layer, the overall single layer, or when the film is a multilayer film, at least one layer, especially the overall layers (excluding a surface oxidized layer if any) should preferably have an extinction coefficient k of at least 0.1, especially at least 0.2 and up to 0.7, especially up to 0.65.

The phase shift film includes at least one layer composed of a silicon base material consisting of silicon and nitrogen or a silicon base material consisting of silicon, nitrogen and oxygen. The silicon base material contains silicon and nitrogen and optionally oxygen. Elements other than these are permissible as long as their amount is at an impurity level. Preferably, transition metals such as molybdenum, zirconium, tungsten, titanium, hafnium, chromium and tantalum are not contained. Use of such silicon base material overcomes the pattern size variation degradation problem associated with transition metal-containing silicon base materials. Also use of such silicon base material provides for improved chemical resistance against chemical cleaning.

In the embodiment wherein the phase shift film is a multilayer film, the thickness of a layer composed of a silicon base material consisting of silicon and nitrogen or a silicon base material consisting of silicon, nitrogen and oxygen is preferably at least 60%, more preferably at least 80% of the total thickness of the phase shift film. If two or more such layers are included, the total thickness of these layers is preferably at least 60%, more preferably at least 80% of the total thickness of the phase shift film. Where a surface oxidized layer is included as will be described later, preferably all the layers of the film excluding the surface oxidized layer are layers composed of a silicon base material consisting of silicon and nitrogen or a silicon base material consisting of silicon, nitrogen and oxygen. Also in the embodiment wherein the halftone phase shift film is a multilayer film, the layer composed of a silicon base material consisting of silicon and nitrogen or a silicon base material consisting of silicon, nitrogen and oxygen may be disposed at any level in the film selected from the side adjacent to the substrate, the side remote from the substrate, and the center in thickness direction.

When the phase shift film is a single layer, the silicon base material of the overall single layer, or when the film is a multilayer film, the silicon base material of at least one layer, especially all the layers (excluding a surface oxidized layer if any) should preferably have a silicon content of at least 30 at %, more preferably at least 40 at %, even more preferably at least 44 at %, and up to 55 at %, more preferably up to 50 at %. In particular, when the phase shift film is a halftone phase shift film having a low transmittance of specifically from 3% to less than 20%, more specifically from 3% to 12%, and even more specifically from 3% to less than 10%, the silicon base material preferably has a silicon content of at least 40 at %, more preferably at least 44 at %, and up to 55 at %, and more preferably up to 50 at %. When the phase shift film is a halftone phase shift film having a high transmittance of specifically from 20% to 30%, the silicon base material preferably has a silicon content of at least 30 at %, more preferably at least 40 at %, and up to 55 at %, and more preferably up to 45 at %.

When the phase shift film is a single layer, the silicon base material of the overall single layer, or when the film is a multilayer film, the silicon base material of at least one layer, especially all the layers (excluding a surface oxidized layer if any) should preferably have a nitrogen content of at least 10 at %, more preferably at least 40 at %, and up to 60 at %, more preferably up to 55 at %. In particular, when the phase shift film is a halftone phase shift film having a low transmittance of specifically from 3% to less than 20%, more specifically from 3% to 12%, and even more specifically from 3% to less than 10%, the silicon base material preferably has a nitrogen content of at least 44 at %, and up to 60 at %, and more preferably up to 56 at %. When the phase shift film is a halftone phase shift film having a high transmittance of specifically from 20% to 30%, the silicon base material preferably has a nitrogen content of at least 10 at %, more preferably at least 40 at %, and up to 60 at %, more preferably up to 55 at %.

When the phase shift film is a single layer, the silicon base material of the overall single layer, or when the film is a multilayer film, the silicon base material of at least one layer, especially all the layers (excluding a surface oxidized layer if any) should preferably have an oxygen content of up to 50 at %, more preferably up to 20 at %, and even more preferably up to 6 at %. In particular, when the phase shift film is a halftone phase shift film having a low transmittance of specifically from 3% to less than 20%, more specifically from 3% to 12%, and even more specifically from 3% to less than 10%, the silicon base material preferably has an oxygen content of at least 0 at %, and up to 6 at %, more preferably up to 3.5 at %, and even more preferably up to 1 at %. When the phase shift film is a halftone phase shift film having a high transmittance of specifically from 20% to 30%, the silicon base material preferably has an oxygen content of up to 50 at %, more preferably up to 20 at %, and in particular, the phase shift film should preferably include at least one layer of a silicon base material having an oxygen content of at least 0 at %, more preferably at least 1 at %.

Suitable silicon base materials include a silicon base material consisting of silicon and nitrogen, i.e., silicon nitride (SiN) and a silicon base material consisting of silicon, nitrogen and oxygen, i.e., silicon oxynitride (SiON).

In order to form a phase shift film as a thin film, a silicon base material with a lower oxygen content is preferred, with an oxygen-free material being more preferred. From this aspect, the phase shift film should preferably include a layer composed of a silicon base material consisting of silicon and nitrogen. In this context, the phase shift film is advantageously a single layer composed of a silicon base material consisting of silicon and nitrogen, or a multilayer film including at least one layer composed of a silicon base material consisting of silicon and nitrogen, especially including at least one layer composed of a silicon base material consisting of silicon and nitrogen and at least one layer composed of a silicon base material consisting of silicon, nitrogen and oxygen.

While the phase shift film may be deposited by any well-known film-forming techniques, the sputtering technique is preferred because films of homogeneity are readily deposited. Either DC sputtering or RF sputtering may be employed. The target and sputtering gas may be selected as appropriate depending on the layer construction and composition of the film. Suitable targets include a silicon target, a silicon nitride target, and a target containing silicon and silicon nitride. The contents of nitrogen and oxygen may be adjusted during reactive sputtering by using nitrogen-containing gas, oxygen-containing gas, or nitrogen/oxygen-containing gas, and optionally carbon-containing gas, as the reactive gas, and adjusting the flow rate thereof. The reactive gas is, for example, nitrogen gas (N₂ gas), oxygen gas (O₂ gas), nitrogen oxide gases (N₂O gas, NO gas, NO₂ gas). It is advantageous from the aspect of reducing a warpage change (ΔTIR) to use a rare gas such as helium, neon or argon gas in the sputtering gas.

According to the invention, a phase shift mask blank as defined above is preferably prepared by sputter depositing a phase shift film on a transparent substrate such that the film may include at least one layer of a silicon base material consisting of silicon, nitrogen and optionally oxygen, and heat treating the phase shift film on the substrate at a temperature of at least 400° C. for at least 5 minutes.

Preferably the step of sputter depositing a phase shift film on a transparent substrate includes one or both of step (A) of depositing a layer composed of a silicon base material consisting of silicon and nitrogen on a transparent substrate by using a sputtering system with a chamber and at least a silicon target, and feeding argon gas and nitrogen gas into the chamber, and step (B) of depositing a layer composed of a silicon base material consisting of silicon, nitrogen and oxygen on a transparent substrate by using a sputtering system with a chamber and at least a silicon target, and feeding argon gas, and at least one gas selected from nitrogen gas, oxygen gas and nitrogen oxide gas into the chamber. In the step of sputter depositing a phase shift film, the flow rate of sputtering gas is preferably adjusted such that the resulting phase shift film may cause a minimal warpage change (ΔTIR) in the phase shift mask blank.

The heat treatment of the phase shift film following deposition is preferably by heating the phase shift film deposited on the substrate at a temperature of at least 400° C., especially at least 450° C. for a time of at least 5 minutes, especially at least 30 minutes. If a transition metal-containing film used as the phase shift film in the prior art is heat treated at such high temperature, precipitates form on the surface and become defects. High temperature is not applicable to the prior art film. In contrast, a phase shift film including at least one layer of a silicon base material consisting of silicon, nitrogen and optionally oxygen does not give rise to the problem associated with the transition metal-containing film, even when such high temperature is applied. The heat treatment temperature is preferably up to 900° C., more preferably up to 700° C. and the time is preferably up to 24 hours, more preferably up to 12 hours. Heat treatment may be performed within the sputtering chamber or after transfer of the substrate to a heat treating furnace apart from the sputtering chamber. The heat treatment atmosphere may be an inert gas atmosphere such as helium gas or argon gas or vacuum. Also an oxygen-containing atmosphere such as oxygen gas atmosphere may be used although a surface oxidized layer is formed on the outermost surface.

In the embodiment wherein the phase shift film is a multilayer film, the film may include a surface oxidized layer as the outermost layer on the surface side (disposed remote from the substrate) in order to suppress any change in quality of the film. The surface oxidized layer may have an oxygen content of at least 20 at %, with even an oxygen content of at least 50 at % being acceptable. The surface oxidized layer may be formed by atmospheric or air oxidation or forced oxidative treatment. Examples of forced oxidative treatment include treatment of a silicon-based material film with ozone gas or ozone water, and heating of a film at 300° C. or higher in an oxygen-containing atmosphere, typically oxygen gas atmosphere by oven heating, lamp annealing or laser heating. The surface oxidized layer preferably has a thickness of up to 10 nm, more preferably up to 5 nm, and even more preferably up to 3 nm. The oxidized layer exerts its effect as long as its thickness is at least 1 nm. Although the surface oxidized layer may also be formed by increasing the amount of oxygen in the sputtering gas during the sputtering step, atmospheric oxidation or oxidative treatment as mentioned above is preferred for forming a less defective layer.

In the phase shift mask blank of the invention, a second film of single layer or multilayer structure may be formed on the phase shift film. Most often, the second film is disposed contiguous to the phase shift film. Examples of the second film include a light-shielding film, a combination of light-shielding film and antireflective film, and an auxiliary film which functions as a hard mask during subsequent pattern formation of the phase shift film. When a third film is formed as will be described later, the second film may be utilized as an auxiliary film (etching stop film) which functions as an etching stopper during subsequent pattern formation of the third film. The second film is preferably composed of a chromium-containing material.

One exemplary embodiment is a phase shift mask blank illustrated in FIG. 2A. The phase shift mask blank depicted at 100 in FIG. 2A includes a transparent substrate 10, a phase shift film 1 formed on the substrate, and a second film 2 formed on the film 1.

The phase shift mask blank may include a light-shielding film as the second film on the phase shift film. A combination of a light-shielding film and an antireflective film may also be used as the second film. The provision of the second film including a light-shielding film ensures that a phase shift mask includes a region capable of completely shielding exposure light. The light-shielding film and antireflective film may also be utilized as an auxiliary film during etching. The construction and material of the light-shielding film and antireflective film are known from many patent documents, for example, Patent Document 4 (JP-A 2007-033469) and Patent Document 5 (JP-A 2007-233179). One preferred film construction of the light-shielding film and antireflective film is a structure having a light-shielding film of Cr-containing material and an antireflective film of Cr-containing material for reducing reflection by the light-shielding film. Each of the light-shielding film and the antireflective film may be a single layer or multilayer. Suitable Cr-containing materials of which the light-shielding film and antireflective film are made include chromium alone, chromium oxide (CrO), chromium nitride (CrN), chromium carbide (CrC), chromium oxynitride (CrON), chromium oxycarbide (CrOC), chromium nitride carbide (CrNC), chromium oxynitride carbide (CrONC) and other chromium compounds.

Where the second film is a light-shielding film or a combination of a light-shielding film and an antireflective film, the light-shielding film is made of a chromium base material having a chromium content of at least 40 at %, especially at least 60 at % and less than 100 at %, preferably up to 99 at %, and more preferably up to 90 at %. The chromium base material has an oxygen content of at least 0 at % and up to 60 at %, preferably up to 40 at %, with an oxygen content of at least 1 at % being preferred when an etching rate must be adjusted. The chromium base material has a nitrogen content of at least 0 at % and up to 50 at %, preferably up to 40 at %, with a nitrogen content of at least 1 at % being preferred when an etching rate must be adjusted. The chromium base material has a carbon content of at least 0 at % and up to 20 at %, preferably up to 10 at %, with a carbon content of at least 1 at % being preferred when an etching rate must be adjusted. The total content of chromium, oxygen, nitrogen and carbon is preferably at least 95 at %, more preferably at least 99 at %, and especially 100 at %.

Where the second film is a combination of a light-shielding film and an antireflective film, the antireflective film is preferably made of a chromium-containing material having a chromium content of preferably at least 30 at %, more preferably at least 35 at % and preferably up to 70 at %, and more preferably up to 50 at %. The chromium-containing material preferably has an oxygen content of up to 60 at %, and at least 1 at % and more preferably at least 20 at %. The chromium-containing material preferably has a nitrogen content of up to 50 at %, more preferably up to 30 at %, and at least 1 at %, more preferably at least 3 at %. The chromium-containing material preferably has a carbon content of at least 0 at % and up to 20 at %, more preferably up to 5 at %, with a carbon content of at least 1 at % being preferred when an etching rate must be adjusted. The total content of chromium, oxygen, nitrogen and carbon is preferably at least 95 at %, more preferably at least 99 at %, and especially 100 at %.

Where the second film is a light-shielding film or a combination of a light-shielding film and an antireflective film, the second film has a thickness of typically 20 to 100 nm, preferably 40 to 70 nm. Also the phase shift film combined with the second film should preferably have a total optical density of at least 2.0, more preferably at least 2.5, and even more preferably at least 3.0, with respect to exposure light of wavelength up to 200 nm.

In the phase shift mask blank of the invention, a third film of single layer or multilayer structure may be formed on the second film. Most often, the third film is disposed contiguous to the second film. Examples of the third film include a light-shielding film, a combination of light-shielding film and antireflective film, and an auxiliary film which functions as a hard mask during subsequent pattern formation of the second film. The third film is preferably composed of a silicon-containing material, especially chromium-free silicon-containing material.

One exemplary embodiment is a phase shift mask blank illustrated in FIG. 2B. The phase shift mask blank depicted at 100 in FIG. 2B includes a transparent substrate 10, a phase shift film 1 formed on the substrate, a second film 2 formed on the film 1, and a third film 3 formed on the second film 2.

Where the second film is a light-shielding film or a combination of a light-shielding film and an antireflective film, the third film may be an auxiliary film (etching mask film) which functions as a hard mask during subsequent pattern formation of the second film. When a fourth film is formed as will be described later, the third film may be utilized as an auxiliary film (etching stop film) which functions as an etching stopper during subsequent pattern formation of the fourth film. The auxiliary film is preferably composed of a material having different etching properties from the second film, for example, a material having resistance to chlorine dry etching applied to the etching of chromium-containing material, specifically a silicon-containing material which can be etched with fluoride gas such as SF₆ or CF₄. Suitable silicon-containing materials include silicon alone, a material containing silicon and one or both of nitrogen and oxygen, a material containing silicon and a transition metal, and a material containing silicon, one or both of nitrogen and oxygen, and a transition metal. Exemplary of the transition metal are molybdenum, tantalum and zirconium.

Where the third film is an auxiliary film, it is preferably composed of a silicon-containing material having a silicon content of preferably at least 20 at %, more preferably at least 33 at % and up to 95 at %, more preferably up to 80 at %. The silicon-containing material has a nitrogen content of at least 0 at % and up to 50 at %, preferably up to 30 at %, with a nitrogen content of at least 1 at % being preferred when an etching rate must be adjusted. The silicon-containing material has an oxygen content of at least 0 at %, preferably at least 20 at % and up to 70 at %, preferably up to 66 at %, with an oxygen content of at least 1 at % being preferred when an etching rate must be adjusted. The silicon-containing material has a transition metal content of at least 0 at % and up to 35 at %, preferably up to 20 at %, with a transition metal content of at least 1 at % being preferred if present. The total content of silicon, oxygen, nitrogen and transition metal is preferably at least 95 at %, more preferably at least 99 at %, and especially 100 at %.

Where the second film is a light-shielding film or a combination of a light-shielding film and an antireflective film and the third film is an auxiliary film, the second film has a thickness of typically 20 to 100 nm, preferably 40 to 70 nm, and the third film has a thickness of typically 1 to 30 nm, preferably 2 to 15 nm. Also the phase shift film combined with the second film should preferably have a total optical density of at least 2.0, more preferably at least 2.5, and even more preferably at least 3.0, with respect to exposure light of wavelength up to 200 nm.

Where the second film is an auxiliary film, a light-shielding film may be formed as the third film. Also, a combination of a light-shielding film and an antireflective film may be formed as the third film. Herein the second film may be utilized as an auxiliary film (etching mask film) which functions as a hard mask during pattern formation of the phase shift film, or an auxiliary film (etching stop film) which functions as an etching stopper during pattern formation of the third film. Examples of the auxiliary film are films of chromium-containing materials as described in Patent Document 6 (JP-A 2007-241065). The auxiliary film may be a single layer or multilayer. Suitable chromium-containing materials of which the auxiliary film is made include chromium alone, chromium oxide (CrO), chromium nitride (CrN), chromium carbide (CrC), chromium oxynitride (CrON), chromium oxycarbide (CrOC), chromium nitride carbide (CrNC), chromium oxynitride carbide (CrONC) and other chromium compounds.

Where the second film is an auxiliary film, the film preferably has a chromium content of preferably at least 40 at %, more preferably at least 50 at % and up to 100 at %, more preferably up to 99 at %, and even more preferably up to 90 at %. The film has an oxygen content of at least 0 at %, and up to 60 at %, preferably up to 55 at %, with an oxygen content of at least 1 at % being preferred when an etching rate must be adjusted. The film has a nitrogen content of at least 0 at %, and up to 50 at %, preferably up to 40 at %, with a nitrogen content of at least 1 at % being preferred when an etching rate must be adjusted. The film has a carbon content of at least 0 at % and up to 20 at %, preferably up to 10 at %, with a carbon content of at least 1 at % being preferred when an etching rate must be adjusted. The total content of chromium, oxygen, nitrogen and carbon is preferably at least 95 at %, more preferably at least 99 at %, and especially 100 at %.

On the other hand, the light-shielding film and antireflective film as the third film are preferably composed of a material having different etching properties from the second film, for example, a material having resistance to chlorine dry etching applied to the etching of chromium-containing material, specifically a silicon-containing material which can be etched with fluoride gas such as SF₆ or CF₄. Suitable silicon-containing materials include silicon alone, a material containing silicon and nitrogen and/or oxygen, a material containing silicon and a transition metal, and a material containing silicon, nitrogen and/or oxygen, and a transition metal. Exemplary of the transition metal are molybdenum, tantalum and zirconium.

Where the third film is a light-shielding film or a combination of a light-shielding film and an antireflective film, the light-shielding film and antireflective film are preferably composed of a silicon-containing material having a silicon content of preferably at least 10 at %, more preferably at least 30 at % and less than 100 at %, more preferably up to 95 at %. The silicon-containing material has a nitrogen content of at least 0 at % and up to 50 at %, preferably up to 40 at %, especially up to 20 at %, with a nitrogen content of at least 1 at % being preferred when an etching rate must be adjusted. The silicon-containing material has an oxygen content of at least 0 at %, and up to 60 at %, preferably up to 30 at %, with an oxygen content of at least 1 at % being preferred when an etching rate must be adjusted. The silicon-containing material has a transition metal content of at least 0 at % and up to 35 at %, preferably up to 20 at %, with a transition metal content of at least 1 at % being preferred if present. The total content of silicon, oxygen, nitrogen and transition metal is preferably at least 95 at %, more preferably at least 99 at %, and especially 100 at %.

Where the second film is an auxiliary film and the third film is a light-shielding film or a combination of a light-shielding film and an antireflective film, the second film has a thickness of typically 1 to 20 nm, preferably 2 to 10 nm, and the third film has a thickness of typically 20 to 100 nm, preferably 30 to 70 nm. Also the phase shift film combined with the second and third films should preferably have a total optical density of at least 2.0, more preferably at least 2.5, and even more preferably at least 3.0, with respect to exposure light of wavelength up to 200 nm.

In the phase shift photomask blank of the invention, a fourth film of single layer or multilayer structure may be formed on the third film. Most often, the fourth film is disposed contiguous to the third film. Exemplary of the fourth film is an auxiliary film which functions as a hard mask during subsequent pattern formation of the third film. The fourth film is preferably composed of a chromium-containing material.

One exemplary embodiment is a phase shift mask blank illustrated in FIG. 2C. The phase shift mask blank depicted at 100 in FIG. 2C includes a transparent substrate 10, a phase shift film 1 formed on the substrate, a second film 2 formed on the film 1, a third film 3 formed on the second film 2, and a fourth film 4 formed on the third film 3.

Where the third film is a light-shielding film or a combination of a light-shielding film and an antireflective film, the fourth film may be an auxiliary film (etching mask film) which functions as a hard mask during subsequent pattern formation of the third film. The auxiliary film is preferably composed of a material having different etching properties from the third film, for example, a material having resistance to fluorine dry etching applied to the etching of silicon-containing material, specifically a chromium-containing material which can be etched with oxygen-containing chloride gas. Suitable chromium-containing materials include chromium alone, chromium oxide (CrO), chromium nitride (CrN), chromium carbide (CrC), chromium oxynitride (CrON), chromium oxycarbide (CrOC), chromium nitride carbide (CrNC), chromium oxynitride carbide (CrONC) and other chromium compounds.

Where the fourth film is an auxiliary film, the film has a chromium content of at least 40 at %, preferably at least 50 at % and up to 100 at %, preferably up to 99 at %, and more preferably up to 90 at %. The film has an oxygen content of at least 0 at % and up to 60 at %, preferably up to 40 at %, with an oxygen content of at least 1 at % being preferred when an etching rate must be adjusted. The film has a nitrogen content of at least 0 at % and up to 50 at %, preferably up to 40 at %, with a nitrogen content of at least 1 at % being preferred when an etching rate must be adjusted. The film has a carbon content of at least 0 at % and up to 20 at %, preferably up to 10 at %, with a carbon content of at least 1 at % being preferred when an etching rate must be adjusted. The total content of chromium, oxygen, nitrogen and carbon is preferably at least 95 at %, more preferably at least 99 at %, and especially 100 at %.

Where the second film is an auxiliary film, the third film is a light-shielding film or a combination of a light-shielding film and an antireflective film, and the fourth film is an auxiliary film; the second film has a thickness of typically 1 to 20 nm, preferably 2 to 10 nm, the third film has a thickness of typically 20 to 100 nm, preferably 30 to 70 nm, and the fourth film has a thickness of typically 1 to 30 nm, preferably 2 to 20 nm. Also the phase shift film combined with the second and third films should preferably have a total optical density of at least 2.0, more preferably at least 2.5, and even more preferably at least 3.0, with respect to exposure light of wavelength up to 200 nm.

The second and fourth films of chromium-containing materials may be deposited by reactive sputtering using a chromium target or a chromium target having one or more of oxygen, nitrogen and carbon added thereto, and a sputtering gas based on a rare gas such as Ar, He or Ne, to which a reactive gas selected from oxygen-containing gas, nitrogen-containing gas and carbon-containing gas is added depending on the desired composition of a film to be deposited.

The third film of silicon-containing material may be deposited by reactive sputtering using a silicon target, silicon nitride target, target containing silicon and silicon nitride, transition metal target, or composite silicon/transition metal target, and a sputtering gas based on a rare gas such as Ar, He or Ne, to which a reactive gas selected from oxygen-containing gas, nitrogen-containing gas and carbon-containing gas is added depending on the desired composition of a film to be deposited.

The mask blank may be processed into a mask by a standard technique. For example, a phase shift mask blank comprising a phase shift film and a second film of chromium-containing material deposited thereon may be processed as follows. First, a resist film adapted for electron beam (EB) lithography is formed on the second film of the phase shift mask blank, exposed to a pattern of EB, and developed in a conventional way, forming a resist pattern. While the resist pattern thus obtained is used as etching mask, oxygen-containing chlorine base dry etching is carried out for transferring the resist pattern to the second film, obtaining a pattern of the second film. Next, while the second film pattern is used as etching mask, fluorine base dry etching is carried out for transferring the pattern to the phase shift film, obtaining a pattern of the phase shift film. If any region of the second film is to be left, a resist pattern for protecting that region is formed on the second film. Thereafter, the portion of the second film which is not protected with the resist pattern is removed by oxygen-containing chlorine base dry etching. The resist pattern is removed in a conventional manner, yielding a phase shift mask.

In another example, a phase shift mask blank comprising a phase shift film, a light-shielding film or a light-shielding film/antireflective film of chromium-containing material deposited thereon as a second film, and an auxiliary film of silicon-containing material deposited thereon as a third film may be processed as follows. First, a resist film adapted for EB lithography is formed on the third film of the phase shift mask blank, exposed to a pattern of EB, and developed in a conventional way, forming a resist pattern. While the resist pattern thus obtained is used as etching mask, fluorine base dry etching is carried out for transferring the resist pattern to the third film, obtaining a pattern of the third film. While the third film pattern thus obtained is used as etching mask, oxygen-containing chlorine base dry etching is carried out for transferring the third film pattern to the second film, obtaining a pattern of the second film. The resist pattern is removed at this point. Further, while the second film pattern is used as etching mask, fluorine base dry etching is carried out for transferring the second film pattern to the phase shift film to define a pattern of the phase shift film and at the same time, removing the third film pattern. If any region of the second film is to be left, a resist pattern for protecting that region is formed on the second film. Thereafter, the portion of the second film which is not protected with the resist pattern is removed by oxygen-containing chlorine base dry etching. The resist pattern is removed in a conventional manner, yielding a phase shift mask.

In a further example, a phase shift mask blank comprising a phase shift film, an auxiliary film of chromium-containing material deposited thereon as a second film, and a light-shielding film or a light-shielding film/antireflective film of silicon-containing material deposited on the second film as a third film may be processed as follows. First, a resist film adapted for EB lithography is formed on the third film of the phase shift mask blank, exposed to a pattern of EB, and developed in a conventional way, forming a resist pattern. While the resist pattern thus obtained is used as etching mask, fluorine base dry etching is carried out for transferring the resist pattern to the third film, obtaining a pattern of the third film. While the third film pattern thus obtained is used as etching mask, oxygen-containing chlorine base dry etching is carried out for transferring the third film pattern to the second film, whereby a pattern of the second film is obtained, that is, a portion of the second film where the phase shift film is to be removed is removed. The resist pattern is removed at this point. A resist pattern for protecting a portion of the third film to be left is formed on the third film. Further, while the second film pattern is used as etching mask, fluorine base dry etching is carried out for transferring the second film pattern to the phase shift film to define a pattern of the phase shift film and at the same time, removing the portion of the third film which is not protected with the resist pattern. The resist pattern is removed in a conventional manner. Finally, oxygen-containing chlorine base dry etching is carried out to remove the portion of the second film where the third film has been removed, yielding a phase shift mask.

In a still further example, a phase shift mask blank comprising a phase shift film, an auxiliary film of chromium-containing material deposited thereon as a second film, a light-shielding film or a light-shielding film/antireflective film of silicon-containing material deposited on the second film as a third film, and an auxiliary film of chromium-containing material deposited on the third film as a fourth film may be processed as follows. First, a resist film adapted for EB lithography is formed on the fourth film of the phase shift mask blank, exposed to a pattern of EB, and developed in a conventional way, forming a resist pattern. While the resist pattern thus obtained is used as etching mask, oxygen-containing chlorine base dry etching is carried out for transferring the resist pattern to the fourth film, obtaining a pattern of the fourth film. While the fourth film pattern thus obtained is used as etching mask, fluorine base dry etching is carried out for transferring the fourth film pattern to the third film, obtaining a pattern of the third film. The resist pattern is removed at this point. A resist pattern for protecting a portion of the third film to be left is formed on the fourth film. Further, while the third film pattern is used as etching mask, oxygen-containing chlorine base dry etching is carried out for transferring the third film pattern to the second film, obtaining a pattern of the second film and at the same time, removing the portion of the fourth film which is not protected with the resist pattern. Next, while the second film pattern is used as etching mask, fluorine base dry etching is carried out for transferring the second film pattern to the phase shift film to define a pattern of the phase shift film and at the same time, removing the portion of the third film which is not protected with the resist pattern. The resist pattern is removed in a conventional manner. Finally, oxygen-containing chlorine base dry etching is carried out to remove the portion of the second film where the third film has been removed and the portion of the fourth film where the resist pattern has been removed, yielding a phase shift mask.

In a photolithographic method for forming a pattern with a half pitch of up to 50 nm, typically up to 30 nm, and more typically up to 20 nm on a processable substrate, comprising the steps of forming a photoresist film on the processable substrate and exposing the photoresist film to light of wavelength up to 200 nm, typically ArF excimer laser (193 nm) or F₂ laser (157 nm), through a patterned mask for transferring the pattern to the photoresist film, the phase shift mask of the invention is best suited for use in the exposure step.

The phase shift mask obtained from the mask blank is advantageously applicable to the pattern exposure method comprising projecting light to the photomask pattern including the pattern of phase shift film for transferring the photomask pattern to an object (photoresist film) on the processable substrate. The irradiation of exposure light may be either dry exposure or immersion exposure. The pattern exposure method is effective particularly when a wafer of at least 300 mm as the processable substrate is exposed to a photomask pattern of light by the immersion lithography with the tendency that a cumulative irradiation energy dose increases within a relatively short time in commercial scale microfabrication.

EXAMPLE

Examples are given below for further illustrating the invention although the invention is not limited thereto.

Example 1

In a chamber of a sputtering system, a 6025 quartz substrate of 152 mm squares and 6.35 mm thick was set. A halftone phase shift film of SiN was deposited by using a silicon target as the sputtering target and argon and nitrogen gases as the sputtering gas, applying a power of 1,900 W across the target, and adjusting the flow rate of argon gas to 17 sccm and the flow rate of nitrogen gas to 40 sccm. Before and after deposition of the halftone phase shift film, a warpage change (ΔTIR) in a central region of 142 mm squares on the substrate surface was measured by a flatness tester Tropel® UltraFlat 200 Mask (Corning Inc.). The absolute value of ΔTIR was 0.22 μm. Thereafter, the substrate was transferred to a heat treating furnace where heat treatment was carried out at 500° C. for 6 hours in an atmosphere containing nitrogen and oxygen gases under substantially atmospheric partial pressures, completing a halftone phase shift mask blank. Before the deposition of the halftone phase shift film and after the heat treatment, the warpage change (ΔTIR) in a central region of 142 mm squares on the substrate surface had an absolute value of 0.10 μm. The halftone phase shift film after heat treatment had a phase shift of 177°, a transmittance of 19%, and a thickness of 60 nm. On X-ray photoelectron spectroscopy (XPS), the film had a Si:N atomic ratio of 46:53.

Next, the halftone phase shift film on the quartz substrate of the halftone phase shift mask blank was completely removed by fluorine base dry etching. Before and after removal of the halftone phase shift film, the warpage change (ΔTIR) in a central region of 142 mm squares on the substrate surface had an absolute value of 0.07 μm.

On a phase shift mask blank having a halftone phase shift film of SiN deposited under the same conditions as above, a second film of Cr-containing material was formed. A resist film for EB lithography was formed on the second film. The resist film was exposed patternwise by EB lithography and developed in a customary manner to form a resist pattern. With the resist pattern made etching mask, the second film was subjected to oxygen-containing chlorine base dry etching to transfer the resist pattern to the second film to form a second film pattern. With the second film pattern made etching mask, fluorine base dry etching was carried out to transfer the second film pattern to the phase shift film to form a phase shift film pattern. Finally the resist pattern and the second film were removed in a customary manner, yielding a phase shift mask. The resulting phase shift mask allowed for correction by a correcting device using electron beam.

Example 2

In a chamber of a sputtering system, a 6025 quartz substrate of 152 mm squares and 6.35 mm thick was set. A halftone phase shift film of SiON was deposited by using a silicon target as the sputtering target and argon, nitrogen and oxygen gases as the sputtering gas, applying a power of 1,900 W across the target, and adjusting the flow rate of argon gas to 15 sccm, the flow rate of nitrogen gas to 40 sccm, and the flow rate of oxygen gas to 2 sccm. Thereafter, the substrate was transferred to a heat treating furnace where heat treatment was carried out at 500° C. for 6 hours in an atmosphere containing nitrogen and oxygen gases under substantially atmospheric partial pressures, completing a halftone phase shift mask blank. Before the deposition of the halftone phase shift film and after the heat treatment, the warpage change (ΔTIR) in a central region of 142 mm squares on the substrate surface had an absolute value of 0.15 μm. The halftone phase shift film after heat treatment had a phase shift of 175°, a transmittance of 24%, and a thickness of 63 nm. On XPS, the film had a Si:N:O atomic ratio of 43:48:8.

Next, the halftone phase shift film on the quartz substrate of the halftone phase shift mask blank was completely removed by fluorine base dry etching. Before and after removal of the halftone phase shift film, the warpage change (ΔTIR) in a central region of 142 mm squares on the substrate surface had an absolute value of 0.18 μm.

Example 3

In a chamber of a sputtering system, a 6025 quartz substrate of 152 mm squares and 6.35 mm thick was set. Sputter deposition was carried out by using a silicon target as the sputtering target and argon, nitrogen and oxygen gases as the sputtering gas, and applying a power of 1,900 W across the target. A layer of SiN was deposited by adjusting the flow rate of argon gas to 15 sccm and the flow rate of nitrogen gas to 40 sccm, and a layer of SiON was then deposited by adjusting the flow rate of argon gas to 10 sccm, the flow rate of nitrogen gas to 40 sccm, and the flow rate of oxygen gas to 10 sccm. There was obtained a halftone phase shift film consisting of the SiN layer and SiON layer. Thereafter, the substrate was transferred to a heat treating furnace where heat treatment was carried out at 500° C. for 6 hours in an atmosphere containing nitrogen and oxygen gases under substantially atmospheric partial pressures, completing a halftone phase shift mask blank. Before the deposition of the halftone phase shift film and after the heat treatment, the warpage change (ΔTIR) in a central region of 142 mm squares on the substrate surface had an absolute value of 0.17 μm. The halftone phase shift film after heat treatment had a phase shift of 177°, a transmittance of 29%, and a thickness of 66 nm. On XPS, the SiN layer had a Si:N atomic ratio of 45:54 and the SiON layer had a Si:N:O atomic ratio of 38:20:41.

Next, the halftone phase shift film on the quartz substrate of the halftone phase shift mask blank was completely removed by fluorine base dry etching. Before and after removal of the halftone phase shift film, the warpage change (ΔTIR) in a central region of 142 mm squares on the substrate surface had an absolute value of 0.19 μm.

Example 4

In a chamber of a sputtering system, a 6025 quartz substrate of 152 mm squares and 6.35 mm thick was set. A halftone phase shift film of SiN was deposited by using a silicon target as the sputtering target and argon and nitrogen gases as the sputtering gas, applying a power of 1,900 W across the target, and adjusting the flow rate of argon gas to 17 sccm and the flow rate of nitrogen gas to 30 sccm. Before and after deposition of the halftone phase shift film, the warpage change (ΔTIR) in a central region of 142 mm squares on the substrate surface had an absolute value of 0.41 μm. Thereafter, the substrate was transferred to a heat treating furnace where heat treatment was carried out at 500° C. for 6 hours in an atmosphere containing nitrogen and oxygen gases under substantially atmospheric partial pressures, completing a halftone phase shift mask blank. Before the deposition of the halftone phase shift film and after the heat treatment, the warpage change (ΔTIR) in a central region of 142 mm squares on the substrate surface had an absolute value of 0.06 The halftone phase shift film after heat treatment had a phase shift of 179°, a transmittance of 7%, and a thickness of 61 nm. On XPS, the film had a Si:N atomic ratio of 47:52.

Next, the halftone phase shift film on the quartz substrate of the halftone phase shift mask blank was completely removed by fluorine base dry etching. Before and after removal of the halftone phase shift film, the warpage change (ΔTIR) in a central region of 142 mm squares on the substrate surface had an absolute value of 0.08 μm.

Comparative Example 1

In a chamber of a sputtering system, a 6025 quartz substrate of 152 mm squares and 6.35 mm thick was set. A halftone phase shift film of SiN was deposited by using a silicon target as the sputtering target and nitrogen gas as the sputtering gas, applying a power of 1,900 W across the target, and adjusting the flow rate of nitrogen gas to 50 sccm. Thereafter, the substrate was transferred to a heat treating furnace where heat treatment was carried out at 500° C. for 6 hours in an atmosphere containing nitrogen and oxygen gases under substantially atmospheric partial pressures, completing a halftone phase shift mask blank. Before the deposition of the halftone phase shift film and after the heat treatment, the warpage change (ΔTIR) in a central region of 142 mm squares on the substrate surface had an absolute value of 0.44 μm. The halftone phase shift film after heat treatment had a phase shift of 175°, a transmittance of 18%, and a thickness of 60 nm. On XPS, the film had a Si:N atomic ratio of 45:54.

Next, the halftone phase shift film on the quartz substrate of the halftone phase shift mask blank was completely removed by fluorine base dry etching. Before and after removal of the halftone phase shift film, the warpage change (ΔTIR) in a central region of 142 mm squares on the substrate surface had an absolute value of 0.45 μm.

Comparative Example 2

In a chamber of a sputtering system, a 6025 quartz substrate of 152 mm squares and 6.35 mm thick was set. A halftone phase shift film of SiN was deposited by using a silicon target as the sputtering target and argon and nitrogen gases as the sputtering gas, applying a power of 1,900 W across the target, and adjusting the flow rate of argon gas to 17 sccm and the flow rate of nitrogen gas to 30 sccm. Before and after deposition of the halftone phase shift film, the warpage change (ΔTIR) in a central region of 142 mm squares on the substrate surface had an absolute value of 0.41 μm. Thereafter, the substrate was transferred to a heat treating furnace where heat treatment was carried out at 300° C. for 6 hours in an atmosphere containing nitrogen and oxygen gases under substantially atmospheric partial pressures, completing a halftone phase shift mask blank. Before the deposition of the halftone phase shift film and after the heat treatment, the warpage change (STIR) in a central region of 142 mm squares on the substrate surface had an absolute value of 0.24 μm. The halftone phase shift film after heat treatment had a phase shift of 179°, a transmittance of 7%, and a thickness of 61 nm. On XPS, the film had a Si:N atomic ratio of 47:52.

Next, the halftone phase shift film on the quartz substrate of the halftone phase shift mask blank was completely removed by fluorine base dry etching. Before and after removal of the halftone phase shift film, the warpage change (STIR) in a central region of 142 mm squares on the substrate surface had an absolute value of 0.27 μm.

Japanese Patent Application No. 2015-072766 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

The invention claimed is:
 1. A phase shift mask blank comprising a transparent substrate of 152 mm squares and 6.35 mm thick and a phase shift film deposited on the substrate and having a phase shift of 150 to 200° with respect to light of wavelength up to 200 nm, wherein said phase shift film includes at least one layer composed of a silicon base material consisting of silicon and nitrogen, has a thickness of up to 70 nm, and provides a warpage change (ΔTIR) of up to 0.2 μm as absolute value in a central region of 142 mm squares on a surface of the substrate between before the deposition of the phase shift film on the substrate and the presence of the phase shift film on the substrate.
 2. The mask blank of claim 1 wherein said phase shift film is a halftone phase shift film composed of a silicon base material consisting of silicon and nitrogen and having a transmittance of 3% to less than 20% with respect to light of wavelength up to 200 nm.
 3. The mask blank of claim 1, further comprising a second film on the phase shift film, the second film being a single layer or a multilayer film composed of a chromium-containing material.
 4. The mask blank of claim 3 wherein the second film is a light-shielding film, a combination of light-shielding film and antireflective film, or an auxiliary film which functions as a hard mask during pattern formation of the phase shift film.
 5. The mask blank of claim 3, further comprising a third film on the second film, the third film being a single layer or a multilayer film composed of a silicon-containing material.
 6. The mask blank of claim 5 wherein the second film is a light-shielding film or a combination of light-shielding film and antireflective film, and the third film is an auxiliary film which functions as a hard mask during pattern formation of the second film.
 7. The mask blank of claim 5 wherein the second film is an auxiliary film which functions as a hard mask during pattern formation of the phase shift film and as an etch stopper during pattern formation of the third film, and the third film is a light-shielding film or a combination of light-shielding film and antireflective film.
 8. The mask blank of claim 5, further comprising a fourth film on the third film, the fourth film being a single layer or a multilayer film composed of a chromium-containing material.
 9. The mask blank of claim 8 wherein the second film is an auxiliary film which functions as a hard mask during pattern formation of the phase shift film and as an etch stopper during pattern formation of the third film, the third film is a light-shielding film or a combination of light-shielding film and antireflective film, and the fourth film is an auxiliary film which functions as a hard mask during pattern formation of the third film.
 10. The mask blank of claim 1 wherein the phase shift film has been sputter deposited and heat treated at 400° C. or above for at least 5 minutes.
 11. A phase shift mask prepared from the phase shift mask blank of claim
 1. 12. A method for preparing a phase shift mask blank of claim 1, comprising the steps of: sputter depositing a phase shift film on a transparent substrate, said depositing step including a step of depositing a layer composed of a silicon base material consisting of silicon and nitrogen on a transparent substrate by using a sputtering system with a chamber and at least one silicon target, and feeding argon gas and nitrogen gas into the chamber, and heat treating the phase shift film on the substrate at 400° C. or above for at least 5 minutes.
 13. A phase shift mask blank comprising a transparent substrate of 152 mm squares and 6.35 mm thick and a phase shift film deposited on the substrate and having a phase shift of 150 to 200° with respect to light of wavelength up to 200 nm, wherein said phase shift film includes at least one layer composed of a silicon base material consisting of silicon and nitrogen, has a thickness of up to 70 nm, and provides a warpage change (ΔTIR) of up to 0.2 μm as absolute value in a central region of 142 mm squares on a surface of the substrate between the presence of the phase shift film on the substrate and after complete removal of the phase shift film from the substrate by etching. 